This assumes you despatched with a serviceable APU! If not you only start with 2 generators!
Remember despatching in the B707 with a generator u/s operating Toronto London. Half way across the pond we lost another generator! At least it was a four engine a/c so we had two left!
Years ago in an unnamed charter company with B737 we had a spate of u/s APUs and ended up going to the Canaries at night too often with no APU. I used to site there halfway between Lisbon and Tenerife thinking what we would do if we lost another generator there and ended up on one. I suppose its all a question of probability!
Mangatete's advice is spot on!